Frame bobbing

In order to do a little "customizing" on my soon to be seriously started project bike, I am thinking about bobbing my frame by cutting off the loop at the rear. (This is a KZ750H.) I am attaching two pictures from the kawasaki website, each marked to roughly show where I might chop with the hacksaw. The red line shows bob #1, and the green line shows bob #2.

The first question is, do I need to weld in or bolt on a piece of steel to replace the structural strength I might lose by hacking off a piece of the frame, or is that loop back there solely for the purpose of supporting the passenger seat (which is being eliminated in my new design)? I have no welding capabilities, and I am not particularly interested in carrying the frame off to somewhere to get it welded.

The second question relates to how much I can safely hack off. Can I cut into the frame web on which the upper shock mount is welded (frame bob #2), or should I cut well behind that (frame bob #1)?

I know some people have cut to the nub of the shock mounts, and probably never even worried about this, but I thought I would ask the experts here.

Here are the pictures:

Frame Bob #1:



Post edited by: apeman, at: 2007/03/11 20:42

Here's Frame Bob #2:

I am not a structural engineer, so my answers are based on experience, not education.
I regularly cut frames and convert them for various friends.
IMO the cuts on the red lines are absolutely safe, and need no replacement bracing.
The cuts on the green lines concern me slightly. A lot depends on what you are doing for rear suspension.
For example, if you are keeping the stock shock and mount, you MAY have a need to add a cross brace to keep it stiff under suspension loads. Probably not, I do emphasize the MAY.
If you are doing a rigid rear (Bobber style), you can probably design the weld on piece replacing the shocks to be your additional brace.
Good luck and keep us informed.

I see what Florida is saying. When you cut at the green there is no cross bracing to prevent that part of the frame from twisting up!

all that part from the green line back is overhang and has no structural value, if you might think it would bend or twist up, wouldn't it do that anyway wether it was cut or not, that part is to support the seat,fender and tail light

hey apeman!

i am about to do the same chop as in pic #2 to my 81 KZ1000 CSR cafe racer project in order to fit a Kaw H2R race tail I have.

i have started welding in some gussets, and am going to run a straight piece between the upper shock mounts and an 'X' inside the frame rails before i chop off the tail.

i figure that straight cross piece between the upper shock mounts will hold everything true and probably put back more stiffness than i am taking out by losing the tail.

i am going to close up the end of the cut "U" material and the open tube but mostly for appearance.

it should happen this week, i'll post pics!









Post edited by: sheik*yerbouti, at: 2007/03/12 00:12

Post edited by: sheik*yerbouti, at: 2007/03/12 00:14

Post edited by: sheik*yerbouti, at: 2007/03/12 00:14

Actually when I look at it again it is quite safe, without any mods, seeing there is the secong cross bar under the seat

kzwolfsr wrote:

Actually when I look at it again it is quite safe, without any mods, seeing there is the secong cross bar under the seat

yea i saw that after the fact as well on the pic, however my 81 1000 CSR doesn't have that so I'm adding betwixt (word of the day) the top shock mount.

funny how it took me 3 edits to post up 3 pics... i suck at getting the url's right and they're on a webserver i run... jeesh... :pinch:

Post edited by: sheik*yerbouti, at: 2007/03/12 00:51

Been there done that! chopped just behind the shock mounts. Don't know if it needed it, but I welded in a crossbrace anyway. Got some of that square street sign pole, with the holes in it as my cross brace. I'll have pics this weekend. Hopefully buttoning everything up this weekend.

Here's a pic of where I chopped mine. I left just enough to save the rearmost seat bolsters, I attached the turn signals to those. I filled in the resulting holes with Bondo to smooth it out and keep crap out so it wouldn't rust.
Welding in an extra brace sure can't hurt, just be careful if you are welding galvanized stock, I've heard the fumes can be toxic..:sick:

Hey Nevan,
Check your PM, I was looking for the thread of your Bobber build.

Frames are welded under tension. Usually in a jig.

If you're gonna be welding in a new crossmember for this project remember to measure and cut your member, then weld it in, and THEN cut the rear of the subframe off. Doing so in the opposite order will give you subframe that is out of whack.

As long as you get a crossmember in there you won't have any flex problems.

Oh, and yes, that part of the frame does get some secondary lashes of torque under hard riding which *can* tweak the subframe. It isn't common on the old bikes with steel tubing, but it isn't impossible either.

Post edited by: caffcruiser, at: 2007/03/12 11:02